Water content controls depth of magma storage under many volcanoes, study finds
A possible key to better eruption predictions overturns conventional theory
Worldwide, 40 to 50 volcanoes are currently erupting or in a state of unrest, endangering hundreds of millions of people. Yet reliable eruption predictions have long eluded scientists, largely because they don’t fully understand why magma starts or stops moving below the surface for weeks, months, or years, before it does eventually break out. The results of a new study could bring them a little closer.
The study reveals that for the most common type of volcano in the world, it is the water content of the magma that controls the depth of temporary storage; the more water there is, the greater the depth. The study challenges the prevailing theory that magma stops rising when its buoyancy equals that of the surrounding rock. A deeper stall may sound like good news, but it doesn’t seem to reduce the risk of an eruption, the scientists say. And, water is the primary driver of explosive eruptions, so when something does eventually break loose in so-called “wet” magma, the results can be extremely violent.
“This study links the depth at which magma is stored to water, which is important because water triggers and largely fuels eruptions,” said lead author Dan Rasmussen, who carried out most of the work. research as a doctoral student. candidate for the Lamont-Doherty Earth Observatory at Columbia University.
In recent years, scientists have used geophysical measurements to determine where magmatic bodies lie beneath many volcanoes; the bodies range from 20 kilometers down to almost the surface. “We already knew where the magma is stored – the final resting place where it builds up before erupting,” said Lamont-Doherty volcanologist Terry Plank, co-author of the paper. “We think it’s slowing down, not stopping. Now we know what the conditions are before an eruption.
The research began in 2015, when Plank suggested Rasmussen pursue the still open question of why storage depth varies from volcano to volcano. The research focused on a particular geological setting: so-called arc volcanoes, located at the intersections of convergent tectonic plates. Arc volcanoes are the most numerous type on the planet, making them the most obvious target for improving forecasting capabilities. With storage depths ranging from 3 to 6 kilometers deep, they comprise the entire “Ring of Fire” surrounding the Pacific Plate, from the Aleutian Islands of Alaska to the South Pacific.
With a team that included study co-author Diana Roman of the Carnegie Institution for Science, Plank and Rasmussen collected ash from eight volcanoes in the remote Aleutians of Alaska. Using ships and helicopters to get around, they encountered rough seas, rough terrain and, on Unimak Island, the threat of giant brown bears.
Ash was the main target because it can contain green olivine crystals, each about 1 millimeter in diameter. Underground, olivine crystals sometimes trap tiny bits of magma as they form. After an eruption sends the crystals to the surface, the magma inside cools and becomes glass. By analyzing the chemical compositions of these tiny pieces of magma, the researchers estimated the water content of the magma.
After estimating the water content of six of the Aleutian volcanoes, the team combined this data with other estimates of magmatic water content from the scientific literature, for a list spanning 62 volcanoes. They supplemented this with data from over 100 other volcanoes around the world.
Rasmussen, who is now a postdoctoral fellow at the Smithsonian Institution’s National Museum of Natural History, said the Smithsonian’s Global Volcanism Program database “was key to compiling these lists because it’s a really good resource for history. eruptions, and we only wanted to consider recently erupted volcanoes. The team focused on recent eruptions because magma reservoirs don’t seem to move much after an eruption. So any estimates of depth or water contents that were made using recently erupted materials have the highest likelihood of accurately reflecting the current state of the volcano’s magma reservoir.
The team ultimately plotted the estimated magma storage depths for 28 volcanoes around the world against their respective estimated magmatic water contents. The results were surprisingly clear: magmas that contain more water tend to be stored deeper in the earth’s crust. The team then showed that water content not only correlates with storage depth, but is responsible for it. They showed this by identifying chemical tracers associated with the formation of water-bearing magmas in the Earth’s mantle, which lies beneath the crust.
As for how water content might determine how deep magma is stored, the authors argue it has to do with a process known as outgassing, in which water dissolved in magma forms bubbles of gas. as it rises through the earth’s crust, and the pressure on the magma decreases – similar to what happens when you slowly unscrew the cap of a seltzer bottle. When the liquid magma begins to degas, it forms crystals and becomes more viscous, that is, less liquid. The researchers suggest that this thickening slows down and immobilizes the magma.
Plank said this makes magma less like, say, beer, and more like toothpaste. But now the newly formed bubbles are trapped in this thickened goo and are trying to expand amid the pressure. It gives the mass an even more explosive potential, should something happen to relieve the pressure and let it all out.
But exactly what something or those things might be is unclear, Plank said. The authors note that the systems they studied are considered “eruptive” – that they “clearly experienced viscous blockage, not slime death, consistent with non-eruptive episodes of blocked intrusions that are commonly observed from years to decades before many eruptions”. In other words, volcanoes are ready to erupt. But the final trigger(s), and how they might be detected, are still largely mysterious. “We know what we found, but we’re not entirely sure of the full implications,” Plank admitted.
Evidence that water content controls the depth of magma storage overturns the most widely accepted explanation in the field today, which holds that magma rises through cracks in the earth’s crust under pressure because rock in fusion is more buoyant than the surrounding solid crust. It then settles to its storage depth as it reaches what is called neutral buoyancy, the point at which the magma is no more buoyant than its surroundings.
The project is linked to a larger Lamont-Doherty effort underway in the Aleutians to plant suites of sophisticated instruments on two highly active volcanoes to document in detail any precursors to eruptions. Plank and others hope the research will lead to the development of better forecasting methods and inexpensive networks that can be used to monitor active volcanoes around the world, many of which are poorly monitored, if at all.
Rasmussen said the next step is to see if these findings hold for volcanoes in other geological settings. These include so-called hotspot volcanoes, like those in the Hawaiian Islands, and rift volcanoes, like those in East Africa. Beyond that, Rasmussen said an even larger question looms: “If the water content of magma controls the depth of magma storage, what controls the water content of magma?”
The study was also co-authored by Mindy Zimmer of Pacific Northwest National Laboratory. Funding and support was provided by the Smithsonian, US National Science Foundation, Community Foundation for Southwest Washington, and US Geological Survey.
This story is based in part on a press release from the Smithsonian Institution.